High speed train scenarios are challenging for User Equipment device (UE) mobility function in that the UE may leave and enter cells rapidly and, therefore, the UE has to quickly detect suitable candidates for handover or cell reselection in order to not lose connection or miss a page or paging signal.
Current 3rd Generation Partnership Project (3GPP) standards have partly taken UE speeds up to 300 kilometers per hour (km/h) into account, but only for the data demodulation part, not for the cell detection. With increased deployment of high speed train lines, increased number of UE users, and increased usage of bandwidth per user, dominating operators are requesting improved UE performance and support for speeds exceeding 300 km/h.
Apart from the relatively shortened time for detecting suitable neighbor cells for handover or cell reselection, high speed may also lead to significant Doppler frequency shifts (or Doppler shift herein) of the received signal. The Doppler shift forces the UE to increase its demodulation frequency when moving towards the cell, and decrease its demodulation frequency when moving away from the cell, relative to the carrier frequency used in the network. The magnitude of the Doppler shift depends on the relative velocity of the UE towards the transmitting antenna, hence with transceivers close to the track, i.e., a small angle between the trajectory of the UE and the line between the UE and the transmitting antenna, a substantial part of the UE velocity will transfer into a Doppler shift. Moreover, there will be an abrupt change of sign of the Doppler shift when the UE passes the transmitting antenna and the smaller the angle, the more abrupt the change is.
The Doppler shift Δf can be expressed as
      Δ    ⁢                  ⁢    f    =      f    ⁡          (                                                  1              -                              (                                  v                  ⁢                                      /                                    ⁢                  c                                )                                                    1              +                              (                                  v                  ⁢                                      /                                    ⁢                  c                                )                                                    -        1            )      where c is the speed of light and v is the relative velocity of the UE towards the transmitting antenna. With an angle α and an actual UE velocity vUE, the relative velocity v towards the transmitting antenna giving rise to Doppler shift becomes v=vUE cos α. How quick the transition is from a negative to a positive shift depends on how far away from the tracks the cell site is located, with lower Doppler shift and less abrupt change if far away and higher Doppler shift and abrupt change in case the cell site is close to the track.
The scenario is illustrated in FIG. 1, where the UE is on a high speed train connected to and moving away from Cell A.2 and quickly needs to detect Cell B.1 towards which it is moving. According to the current 3GPP standards, the cell site can be as close as 2 meters (m) from the tracks. The train may travel at speeds up to 450 km/h and the UE is handed over, or has to reselect cells to camp on, frequently.
Handover to a new Primary Cell (PCell), configuration of a new Secondary Cell (SCell), and configuration and activation of a new Primary Secondary Cell (PSCell) is usually based on measurement reports from the UE, where the UE has been configured by a network node to send measurement reports periodically, at particular events, or a combination thereof. The measurement reports contain physical cell Identity (ID), reference signal strength (Reference Signal Received Power (RSRP)) and reference signal quality (Reference Signal Received Quality (RSRQ)) of the detected cells.
Cell detection in 3GPP Long Term Evolution (LTE) systems, aiming at detecting and determining cell ID and cell timing of neighbor cells, is facilitated by two signals that are transmitted in each Enhanced Universal Terrestrial Radio Access Network (EUTRAN) cell on a 5 millisecond (ms) basis: the Primary and the Secondary Synchronization Signal (PSS and SSS, respectively). Moreover, Reference Signals (RSs) are transmitted in each cell in order to facilitate cell measurements and channel estimation.
The PSS exists in three versions, one for each out of three cell-within-group IDs, and is based on Zadoff-Chu sequences that are mapped onto the central 62 subcarriers and bordered by five unused subcarriers on either side. There are in total 168 cell groups, and information on to which cell group a cell belongs is carried by the SSS, which is based on m-sequences. This signal also carries information on whether it is transmitted in subframe 0 or subframe 5, which is used for acquiring frame timing. For a particular cell, the SSS is further scrambled with the cell's cell-within-group ID. Hence, in total there are 2×504 versions, two for each out of 504 physical layer cell IDs. Similar to PSS, SSS is mapped onto the central 62 subcarriers and bordered by five unused subcarriers on either side. The time (subframe)-frequency (subcarrier) grid or layout of synchronization signals in a 3GPP LTE Frequency Division Duplex (FDD) radio frame is shown in FIG. 2. Subframes 1-3 and 6-8 may be used for Multi-Broadcast Single-Frequency Network (MBSFN) or may be signaled to do so for other purposes, by which a UE cannot expect reference signals in more than the first Orthogonal Frequency Division Multiplexing (OFDM) symbol. The Physical Broadcast Channel (PBCH) (carrying Master Information Block (MIB)) and synchronization signals are transmitted at prior known OFDM symbol positions over the central 72 subcarriers.
Detection by a UE of a cell is, as is well-known in the art, based on matched filtering using the three PSS versions over at least five ms of received samples. Correlation peaks in the filter output may reveal synchronization signals from one or more cells. This is referred to as symbol synchronization.
Upon having established symbol synchronization and identified the cell-within-group ID, the next step is SSS detection to acquire frame timing and physical layer cell ID. After decoding the SSS, the cell group ID and thereby the full physical layer cell ID is acquired. Moreover, frame timing and cyclic prefix configuration are determined.
The pair of PSS and SSS is always transmitted from the same antenna port, but different pairs may be transmitted from different antenna ports (3GPP Technical Specification (TS) 36.211 V12.3.0, Section 6.11).
Existing methods of cell detection at a UE include:                Non-coherent PSS detection, where matched filtering is carried out individually for each receiver branch, and then the signal magnitudes (potentially squared to powers) of all receiver branches are added before peak detection is carried out.        Coherent SSS detection, where after having established where the PSS is located in time, the same PSS is used for estimating the radio channel for the cell-to-be-detected before coherently adding the SSS from the different receiver branches and carrying out decoding.        Non-coherent SSS detection, where the timing information from PSS is used but no radio channel is estimated based on it.        
Furthermore, either method may also include interference cancellation of partially or fully overlapping signals from already detected cells, whereby the prior known signals are subtracted before carrying out the detection of PSS or decoding of SSS, see for instance commonly owned and assigned International Publication No. WO 2014/135204 A1 entitled CHANNEL ESTIMATION FOR INTERFERENCE CANCELLATION.